He was born in Kilkenny in Ireland. His original name is unknown: he was adopted as a child and only took the name James Mason when he and his family moved to the United States of America in 1861. There he learnt chess and eventually secured a job at the New York Herald.

Mason made his first mark on the chess scene in 1876, when he won the Fourth American Congress, the New York Clipper tournament, and defeated William Bird in a match by the comfortable margin of 13-6. In 1878 he settled in England. His best tournament results were third at the very strong Vienna 1882 tournament, third at Nuremberg 1883 and equal second at Hamburg 1885. At the strong Hastings 1895 tournament he finished joint twelfth with 9.5/21. Mason wrote several books on chess, the most popular being The Principles of Chess in Theory and Practice (1894) and The Art of Chess (1895).

The opening 1.d4 d5 2.Bf4 (in algebraic notation) is sometimes called the Mason Variation in his honour; he played it several times from the 1880s. The variation of the King´s Gambit 1.e4 e5 2.f4 exf4 3.Nc3 (allowing 3...Qh4+) is sometimes called the Mason Gambit, though Mason lost the only game he played with it (against Samuel Rosenthal at Paris 1878); it is also known as the Keres Gambit.